It is known, notably from WO2015/096955, to convert luminance values YLDR of colors of a low dynamic range (LDR) version of an image into expanded luminance values YHDR for generating colors of a corresponding high dynamic range (HDR) version of this image, by scaling exponentially these low dynamic range luminance values YLDR (or low pass filtered luminance values YLDR-LPF) using an exponent function with an exponent. For each pixel p of the LDR image, a value of the exponent EHDR(p) can be computed, for instance as disclosed in WO2015/096955, WO2017/032822 and WO2017/036908. Values of this exponent EHDR(p) for each pixel of the image forms an expansion exponent map.
Such an exponential inverse Tone Mapping (iTM) can then be summarized by the following equation:YHDR(p)=Y′LDR(p)EHDR(p)×YS(p)   (1a)    where Y′LDR(p) can be equal to the low dynamic range luminance value of the color of the pixel p of the image, namely YLDR(p) or, for instance, to a low pass filtered luminance value of the same color as disclosed for instance in WO2017/190850;    where YS(p) is a scaling function, for instance adapted as disclosed in WO2015/096955 to enhance details that may have been smoothed by the luminance expansion, and/or for denoising the expanded version of the image.
A LDR version of an image is generally provided such that the range of luminance values of its colors are all inferior or equal to a low peak luminance level LLDR which correspond to the maximum luminance that a LDR display device can display. If such a LDR version is coded using BLDR bits, the low peak luminance level LSDR corresponds to the coded value 2BLDR−1. If BLDR=10 and if LLDR=100 nits, the coded value 1023 corresponds to 100 nits.
As generated for instance as described above, a HDR version of an image is such that the range of luminance values of its colors are all inferior or equal to a higher peak luminance level LHDR which correspond to the maximum luminance that a given HDR display device can display. It means that a given HDR version of an image is specifically adapted to be displayed by display devices having a peak luminance level equal to LHDR. If such a HDR version is coded using BHDR bits, the high peak luminance level LHDR corresponds to the coded value 2BHDR−1. If BHDR=12 and if LHDR=1000 nits, the coded value 4095 corresponds to 1000 nits.
A problem arises when an image has to be displayed by a target display device having a target peak luminance LT different from the low peak luminance level LLDR and different from the high peak luminance level LHDR, whenever LLDR<LT<LHDR) or LT>LHDR. Namely, this problem is the tone adaptation of an image to a target peak luminance LT different from the high peak luminance level LHDR for which the HDR image has been or can be generated.
The document WO2013/046095 discloses several solutions to solve this problem.